Setting Systematic random sampling was used to select 18 schools in urban districts.

Participants Children were followed up over 5 years with an assessment in each year. Consent, from both adolescents and their parents,
was required. At baseline, 759 students were recruited into the cohort, and of these students, 740 remained in the cohort
for the first round, 712 for the second round, 630 for the third round and 585 for the last round of follow-up.

Primary and secondary outcome measures Anthropometric measurements were taken using established guidelines. Six main groups of exposure factors including dietary
intake and behaviours, physical activity and sedentary behaviours, family social and physical environment, school environment,
socioeconomic status and parental characteristics were measured.

Results Retention rate was high (77%). Within 5-year period, the prevalence of combined overweight and obesity using International
Obesity Task Force cut-off values increased from 14.2% to 21.8%. Time spent on physical activity decreased significantly in
the 5-year period from 87 to 50 min/day. Time spent on sedentary behaviours increased in the 5-year period from 512 to 600 min/day.

Conclusions The complete data analysis of this cohort study will allow a full exploration of the role of environmental and lifestyle
behaviours on adolescent overweight and obesity and also identify the factors most strongly associated with excess weight
gain and the appearance of overweight and obesity in different age groups of adolescents from this large city in Vietnam.

Contributors TNHHD, HKT and MJD all contributed to the conceptualisation, design and management of the study and all revised this manuscript
critically. All authors gave final approval of the version to be published.

Funding This work was supported by the Nestlé Foundation grant (P673-1773).

Data sharing statement Access to the data set is available from the corresponding author (nguyenhoang_doantrang{at}yahoo.com) or the principle investigator (hongutc{at}yahoo.com) in STATA format for academic researchers interested in undertaking a formally agreed collaborative research project.